Many thermoplastics have a natural, high gloss finish when injection molded, particularly compositions containing styrenic polymers such as emulsion ABS polymers. For many applications, high gloss is a very desirable characteristic and it may be one of the most important factors in the selection of the material. On the other hand, for many other products, such as automotive and business equipment, for example computer and other electronic equipment, there is a trend toward matte or low gloss finishes, principally for aesthetic reasons.
One technique for obtaining low gloss is to use a textured mold surface. Textured molds are even used to mold low gloss materials in order to further accentuate the dull finish. Using a high gloss product in a textured mold does not provide optimum results because the parts are not uniform over a long run. The mold surface tends to pick up material in different areas resulting in varying degrees of gloss over the surface of the parts.
Some polymers, like polypropylene, naturally exhibit less gloss than other thermoplastic polymers. Other polymers can easily be made to exhibit low gloss by the introduction of one or more additional constituents. An example of such a polymer is high impact polystyrene which is made by mixing polybutadiene with polystyrene. The large domains of rubber within the polystyrene phase result in a low gloss material. Still another inherently low gloss thermoplastic is ABS graft polymer made by the bulk polymerization process. The latter process involves the dissolving of polybutadiene in styrene monomer and the subsequent graft reaction between the styrene and the polybutadiene together with acrylonitrile monomer. At the end of the reaction, relatively large particles of rubber are created which may contain some occluded styrene-acrylonitrile copolymer. The large particle size of these rubbery domains are believed to be responsible for the low gloss surface finish of such products.
Approaches to obtaining low gloss for materials which naturally exhibit high gloss include functionalizing the butadiene or butadiene/styrene backbone or substrate onto which the other monomers are grafted. U.S. Pat. No. 4,612,347, for example, describes an acid functionalized graft copolymer. Other methods copolymerize a glycidyl ester with acrylonitrile, a vinyl and a rubbery copolymer as described in Japanese Kokai No. 63-156847. U.S. Pat. No. 4,710,534 suggests the use of an olefin rubber in combination with ABS in a polycarbonate or polycarbonate/polybutylene terephalate blend. U.S. Pat. Nos. 4,677,162, 4,624,986 and 4,526,926 all achieve low gloss by using mass or bulk polymerized ABS. U.S. Pat. No. 4,761,463 describes the use of vinyl compounds containing glycidyl esters of unsaturated acids for low gloss surfaces and U.S. Pat. No. 4,554,316 uses epoxy and acid (anhydride) functional olefins for the same purpose.
The use of diepoxides to stabilize ABS/PBT blends is the subject of copending U.S. patent application Ser. No. 07/318,888 filed Mar. 6, 1989, by Jalbert and Cox and assigned to the same assignee as this application. The diepoxides are used for a different purpose in a different polymer alloy and there is nothing to suggest their use as additives to achieve low gloss in ABS/polycarbonate blends.